1. Field of the Invention
The invention generally relates to surge protection of coaxial cables and transmission lines. More particularly, the invention relates to a compact surge protector with a high current capacity, for use in-line with a coaxial cable or transmission line, configurable for operation in a range of different frequency bands.
2. Description of Related Art
Electrical cables, for example coaxial transmission lines of antenna towers, are equipped with surge suppression equipment to provide an electrical path to ground for diversion of electrical current surges resulting from, for example, static discharge and or lightning strikes.
Prior coaxial suppression equipment typically incorporated a frequency selective shorting element between the inner and outer conductors dimensioned to be approximately one quarter of the frequency band center frequency in length, known as a quarter wavelength stub. Therefore, frequencies within the operating band pass along the inner conductor reflecting in phase from the quarter wavelength stub back to the inner conductor rather than being diverted to the outer conductor and or a grounding connection. Frequencies outside of the operating band, such as low frequency surges from lightning strikes, do not reflect and are coupled to ground, preventing electrical damage to downstream components and or equipment.
Depending upon the desired frequency band, a shorting element dimensioned as a quarter wavelength stub may have a required dimension of several inches, requiring a substantial supporting enclosure. Where the supporting enclosure and any necessary interface to the surge suppressor body are not machinable along a single longitudinal axis of the surge suppressor body, additional manufacturing costs are incurred. Prior quarter wavelength stub surge suppressors, such as described in U.S. Pat. No. 5,982,602 “Surge Protector Connector” by Tellas et al, issued Nov. 9, 1999 commonly owned with the present application by Andrew Corporation and hereby incorporated by reference in the entirety, are largely machinable along a single longitudinal axis of the surge suppressor body and also reduce the required enclosure size by spiraling the shorting element away from the inner conductor to a nearly full circumference loop around the inner conductor.
However, because the shorting element requires sufficient cross sectional area to carry the desired surge current load, and requires a suitable separation from the other elements to prevent flashover during a surge condition, the required enclosure is still relatively large and necessarily introduces a significant variation to the outer conductor diameter as it passes along the body of the surge suppressor. Variations in the outer conductor diameter introduce an impedance discontinuity that increases insertion losses. Also, the shorting element is coupled to the outer conductor via a slidable slot connection, secured by a screw that increases manufacturing complexity and also introduces a weak point in the electrical interconnection with the outer conductor.
Alternative shorting elements in other prior surge suppressors include a single planar spiral with multiple loops that requires an increased body diameter to maintain the required spacing between the loops. Similarly, a helical coil shorting element configuration is expensive to manufacture with precision and requires a significant extension of the longitudinal dimension of the surge suppressor.
The spiral aspect of the shorting element is an inductor structure that increases the inductance of the shorting element. The high frequency magnetic field effects of an inductor structure having an affect on the impedance of the frequency selective shorting element that allows the overall length of the shorting element to be reduced, compared to a straight or minimally spiraled quarter wavelength stub. Precision manufacture by machining or bending of a range of different spiral inductor shorting element configurations, to allow supply of a surge suppressor optimized for each of a range of different frequency bands, adds a significant manufacturing cost and lead time to the resulting family of surge suppressors.
Competition within the electrical cable, connector and associated accessory industries has focused attention on cost reductions resulting from increased manufacturing efficiencies, reduced installation requirements and simplification/overall number of discrete parts reduction.
Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art.